Metal oxide surge arresters are electrical devices installed in electrical grids in order to protect other electrical apparatuses from the consequences arising of destructive over voltages. Such consequences may result in damages of the electrical system as well as of its components. The working principle is based on a strongly nonlinear characteristic of the resistivity of metal oxide resistors as a function of the applied voltage. This allows a surge arrester to limit the damaging effects of a lightning-effected over voltage by draining currents of many kA to ground for a short time. In comparison, a surge arrester has, under normal service conditions, a leakage current of parts of mA over years of operation.
The maximum continuous voltage Uc defines the condition under which the arrester can work indefinitely. An elevated voltage higher than Uc can be applied for a limited time, which is specified by the manufacturer. Exceeding this specified time will cause an overload, which causes the Metal Oxide surge arrester to reach a thermal limit and to fail, resulting in a short circuit fault and in a permanent damage of the surge arrester.
This failure case is recognized by the international standards IEC 60099-4 and IEEE C62.11a by specification of a short circuit test. According to the test procedure, in order to prevent damages on the equipment installed close to the surge arrester in the substation, the surge arrester has to provide a failure mode without violent shattering of the housing, and shall be able to self-extinguish open flames within 2 minutes after the end of the test.
The problem of conventional assemblies for protecting an electrical grid line against temporary overvoltages resides in that the surge arrester suffers irreversible damage in case of a temporary overvoltage in the electric line lasting longer than a few tenths of milliseconds, e.g. longer than 100 ms extending over a few cycles up to several seconds or more, because the surge arrester suffers a thermal overload. The temporary overvoltage is referred to as TOV hereinafter such as known of IEC 60099-4:2014; edition 3.0, for example. The same standard defines impulse voltages with times lasting shorter than a few milliseconds e.g. shorter than 100 ms.
In regions having high fire hazards like Australia and some arid areas of the United States, additional technical specifications have set more severe requirements for reducing the risk of ignition of a fire: Additional to the normal requirements stated by IEC or IEEE, a surge arrester has to fail without spreading hot particles having enough energy to cause a fire in its surroundings.
This is proven by carrying out a short circuit test with the arrester mounted at a defined height to ground, wherein the ground has been previously covered with a thermal sensitive material that is easily inflammable. For example, Australia standard AS 1307.2 specifies many thin calibrated paper layers on the ground, while USA (Cal fire) specifies a fuel bed comprising dry grass, prepared with fuel.
Previous technical solutions for the protection from fire promotion by a surge arrester are mainly based on the concept of limiting the effect of the arc burning between upper and lower terminals of the surge arrester in case of a fault current. The consequence is that while the surge arrester is overloaded during testing (and later in the field), the overload causes a short circuit failure, and an arc is subsequently burning between the surge arrester terminals. The terminals are equipped with especially developed electrodes, which shall force the arc to move, thereby limiting the size of the melted metal droplets falling to ground.
For example, EP1566869 B1 discloses a shaped-electrode-concept for arc guiding in a surge arrester.
In view of the above problems the protection of the environment against unintended fire caused by a current overload shall be improved.